1. Field of the Invention
This invention relates to pipeline cleaning pigs. More particularly it relates to pipeline cleaning pigs having one or more slotted flexible discs.
2. Description of the Related Art
The buildup of coatings on the interior of surfaces of pipelines is a common problem in the chemical, petroleum, and water supply industries. Pipelines carrying petroleum products may sustain buildups of paraffin, asphaltene, or other substances which may adhere to the interior surface of the pipeline. If the buildup of the coating material remains unchecked, the flow capacity of the pipeline may be severely restricted.
For a number of years, pipeline operators have used pigs to remove undesirable coatings from the interior of pipelines. These prior art pigs typically comprise a metal body or mandrel that supports one or more flexible scraping discs, and/or cups. The discs are ordinarily made of some type of rubber or thin metal. There are several disadvantages associated with these prior art pigs. First, if the metal mandrel pig suffers a catastrophic failure while inside the pipeline, the metal fragments from the mandrel may become lodged in the wall of the pipeline or in valves or pipeline junctions, or they may damage downstream equipment such as pumps or sensors. The metal mandrel in combination rubber metal pigs presents a further disadvantage. In certain pipeline settings, it may be necessary for the pig to be able to pass through relatively extreme radius bends. In such extreme radius bends, the relatively rigid metal mandrel may prevent the pig from successfully navigating the bend. There are some metal mandrel pigs that have a universal joint in the mandrel that will enable the pig to pass a bend. However, the universal joints add costs, and present another mechanism that is subject failure within the pipeline.
Completely nonmetallic pigs do not suffer from the foregoing disadvantages. For example, if a nonmetallic pig suffers a catastrophic failure inside the pipeline, the rubber fragments will ordinarily degrade over time in the presence of the flowing fluid. In addition, a second pig may be sent through the pipeline to either destroy or dislodge the rubber fragments. While the nonmetallic pigs do not present the disadvantages associated with the metal mandrel pigs, they nevertheless present a further disadvantage, that becomes readily apparent in pipelines of variable internal diameter.
For ease of cleaning and maintenance, it is ordinarily desirable for a pipeline to have a constant internal diameter. However, there are many circumstances where two sections of pipeline, each having a different internal diameter, are joined together. In such circumstances, a pig having a given diameter may be satisfactory to clean the interior of one of the pipeline sections, but not the other. For example, the cleaning discs and/or cups on the all-rubber pig may not be sufficiently flexible to enable the pig to readily move from a relatively larger diameter pipe length to a relatively smaller diameter pipe. For those pigs that do have sufficiently flexible cleaning discs, there is the further risk that, as the pig encounters a reduced internal diameter pipe section, and the discs are folded backward, buckling may occur. As the discs buckle, the peripheral surfaces of the discs will have a tendency to form folds and ripples, not unlike the folds that form in a piece of cloth pressed through a gun barrel during cleaning. The buckling is a natural consequence of the overgauged discs being compressed into the undergauged internal diameter of the second section of pipe.
There is a further disadvantage associated with current pigs. The discs on current pigs ordinarily have smooth surfaces. However, there may be circumstances where the undesirable coating has become particularly hard and, therefore, resistant to removal by a smooth surfaced pig.